According to the the prior art, plastic filaments are laid in thread groups, using an automated mechanism, onto a plastic film previously put on a convex and rigid mould, and subsequently they are covered by a second film having the same features as the first one. A considerable drawback of known production processes lies in the complexity of the automated plant, comprising the rigid adjustable mould and the robot which puts the reinforcement filaments on the first plastic laminate laid onto the rigid convex mould. In order to allow the adjustment of the mould, the latter must be provided with a plurality of flexible fillets or strips, which are nevertheless substantially rigid and which are provided with associated pneumatic cylinders to be adjusted individually depending on the value of the sail's camber.
Further problems must be faced when a different mould must be constructed for a sail of different size.